Right now, Claude Opus 4.5 does not have official “Structured Outputs / JSON mode” support in the Anthropic API. According to the Structured Outputs documentation, the feature is in public beta only for Claude Sonnet 4.5 and Claude Opus 4.1, activated via the structured-outputs-2025-11-13 beta header. Opus 4.5 is not listed as a supported model for schema-validated JSON at this time. That means you should not expect strict JSON Schema compliance when calling Opus 4.5, even though it can often produce JSON-looking text.
In practice, you can still get Opus 4.5 to emit JSON by prompting carefully (“Return only valid JSON matching this schema …”), and for many use cases this works well enough. However, without the Structured Outputs feature, you must treat the output as untrusted: you should run it through JSON parsing, validation, and possibly repair logic. In production systems, it’s common to wrap Opus 4.5 with a small post-processor that retries on parse errors or validates required fields before passing results downstream. If your use case absolutely requires guaranteed schema-correct JSON, Anthropic’s own docs recommend using one of the models officially supported by Structured Outputs (currently Sonnet 4.5 or Opus 4.1).
For workflows where you still want Opus 4.5’s reasoning but also need structured data, a good pattern is to combine it with a retrieval layer and a vector database such as Milvus or Zilliz Cloud. For example, you can ask Opus 4.5 to extract entities from text in a semi-structured way, then validate/normalize them in your own code and store clean representations as vectors in Milvus. Later, your system can use those embeddings for semantic search or agent memory, while still keeping a strict typed schema at the database boundary. This shifts the burden of strict structure away from the model and into your own code and storage, where you have full control.